Why are laser linewidths in frequency domain?

Question

I am learning about the lasers and I find one thing confusing about the lasers.

I understand that there is a range of wavelength laser, it is never exactly monochromatic. So, I understand the concept of FWHM to define a linewidth.

What I have difficulty understanding is this: Why is it often stated in terms of frequency (i.e., kHz, MHz, etc) and not wavelength (in nm)? Is it not more intuitive to work in the wavelength range?

Also, I am confused about some terminology itself. Sources often write "narrow linewidth", but when they also for the opposite, say "high bandwidth". Would it not be better to call it "broad linewidth"?

Answer 1

Thanks to the constant speed of light in a vacuum, we have $c = \lambda f$, so there is a direct relation between frequency and wavelength. This allows us to use those values interchangeably. Moreover, the energy of a given photon, which is related to the frequency $f$ of the field, not only does not depend on the medium in which light propagates but also depends on it linearly ($E=h f$), making that value sometimes more intuitive to use. However, very often, the choice of a specific unit of energy depends on what is common standard among a given community. For example, in the field of spectroscopy, you often see that the energy of photons is presented in units of $\text{cm}^{-1}$, i.e., $1/ \lambda$.

Regarding your second question, I haven't encountered anyone referring to broad lasers as high-bandwidth sources, as that term typically applies to systems processing various signals. Nonetheless, the meaning still should suggest that the source is broad.